This invention relates to a method for the manufacture of reduced pellets, which method comprises heat-treating anthracite at temperatures between 600.degree. C and 1000.degree. C, pulverizing the heat-treated anthracite to give rise to a carbonaceous reducing material, mixing the carbonaceous reducing material with a powdered raw metallurgical ore such as of iron, chromium or manganese, pelletizing the resultant mixture and thereafter roasting the pellets.
It has already been known to the art to obtain reduced pellets by combining a powdered ore, a powdered carbonaceous reducing material and a binder, pelletizing the resultant mixture and roasting the pellets in a furnace such as a rotary kiln. In the case of reduced pellets which are produced from iron ore as the starting material, they are used in their unmodified form as reduced iron. In the case of reduced pellets produced by using either chromium ore or manganese ore as the starting material, the reduced pellets are further refined in an electrical furnace to produce a corresponding ferroalloy. The primary objects of the production of reduced pellets from such a mineral ore are as follows. In the first place, a raw mineral ore which is in a powdery form, does not provide convenience of handling when used as a raw material for refining, whereas the same ore provides convenience of handling when it is used in a pelletized form. Since the pellets are already in a partially reduced state, the additional refining to be carried out in an electric furnace enjoys enhanced productivity. Further, in the production of a ferroalloy, the refining is generally effected by charging the electric furnace with the raw ore in conjunction with carbon. In the refining of this form, there is entailed a heavy consumption of expensive electric power. If, in this case, the refining is carried out by preparing reduced pellets in advance and thereafter charging the electric furnace with these reduced pellets, then the overall cost of energy consumption can be lowered because the pellets have already been roasted to be reduced by the combustion of heavy oil or the like and, therefore, the electric power actually consumed in the electric furnace is less than would otherwise be consumed, the saving being ascribable to the use of less expensive heavy oil in said reduction.
In the preparation of reduced pellets, coke from coal is generally used as the carbonaceous reducing material. For use in the preparation of reduced pellets, the carbonaceous reducing material is required to be in a finely ground state. Since the coke from coal has extremely poor grindability, the work of finely grinding this coke requires enormous power. The lining parts of the grinding machine, for example a ball mill, in which the coke is treated therefore experience heavy wear. Moreover, in the machinery employed indispensably in handling the coke from coal, accelerated wear occurs such as on parts of the machines in the conveyor system, rabbit dryer vanes in the drying system, etc. Thus, the work of size reduction turns out to be guite costly.
Anthracite is also known as a carbonaceous reducing material usable in the preparation of reduced pellets. For the purpose of this use, however, anthracite has its merits and demerits. As for merits, anthracite enjoys excellent grindability and avoids causing accelerated wear on the machinery. Table 1 shows the properties of coke from coal and of anthracite.
Table 1. ______________________________________ Properties of coke from coal and anthracite Anthracite A Anthracite B Coke from (produced in (produced in Item coal South Africa) China) ______________________________________ 1 Fixed carbon 88 wt% 76 wt% 83 wt% 2 Volatile matter 2 12 9 3 Ash content 10 12 8 4 Fuel ratio 44.0 6.3 9.2 5 Grindability(A) 82 KWH/t 22 KWH/t 33 KWH/t 6 Wear (B) 2.0% 0.08% 0.5% ______________________________________
The term "fuel ratio" given in the foregoing table means the quotient obtained by dividing the content (% by weight) of fixed carbon by the content (% by weight) of volatile matter. The grindability (A) represents the value of work index which is defined by Fred C. Bond in "British Chemical Engineering" (June 1961), 6 p. 378 and which is also defined in "Testing Method of Grinding Work Index," M 400, 1969 of Japanese Industrial Standards. It indicates the degree of grindability of a given carbonaceous reducing material. The value W.sub.i was obtained by grinding a fraction of a 700-ml sample in a small test mill measuring 305mm in diameter and 305mm in height and operated at a revolution number of 70rpm, sifting the resultant powder with a sieve of 149 .mu.m (P.sub.1) to have the powder divided into a stopped portion remaining unpassed on the sieve and a passed portion collecting under the sieve, adding another fraction of said sample in the same amount as that of the passed portion to the stopped portion on the sieve, repeating the sifting-adding step until the passed portion obtained under the sieve reached a constant weight and performing a calculation on the found weight in accordance with the following formula: ##EQU1## wherein, P.sub.1 denotes the mesh size (.mu.m) of the sieve used in the test grinding G.sub.bp denotes the net ground weight (in grams) per the one rotation of the test mill in the grinding test mill, F.sub.80 denotes the particle size of 80% pass (in .mu.m) of the sample and P.sub.80 denotes the particle size of 80% pass (in .mu.m) of the ground product.
B denotes the wear of the carbonaceous reducing material, which was obtained as the result of the wear test described herein below. In a bed of a given carbonaceous reducing material packed in the form of a cylinder measuring 100mm in depth and 220mm in inside diameter, four test pieces of mild steel each weighing about 140g and measuring 90mm .times. 10.2mm Diam. were fixed in position in such a way that they would be exposed to uniform contact with the particles of carbonaceous reducing material within the bed and the cylindrical container was rotated at the rate of 1500rpm for 3 hours. At the end of the rotation, the test pieces were weighed to find loss of weight. And the found loss of weight was noted in terms of percentage. The value thus obtained serves as a criterion for estimation of the extent to which said carbonaceous reducing material would cause wear on the machinery in use.
In Table 1, it is seen that anthracite has different properties of grindability (A) and wear (B) from those of coke from coal. To be specific, the two grades of anthracite showed decidedly lower values of KWH per ton and far smaller percentages of wear than those of the coke, indicating that the substance anthracite possesses properties desirable for use as the material for reduced pellets. When reduced pellets are to be prepared by using anthracite as the raw material, however, there inevitably ensues a disadvantage that the formed pellets are disintegrated in the course of roasting or, if they fortunately escape this disintegration, they suffer from deficient tenacity. The main reason for this disadvantage is that the volatile component of anthracite escapes and the anthracite itself expands or shrinks while the anthracite is exposed to intense heat and, consequently, the formed pellets fail to retain their strength during or after the roasting. This disadvantage manifests itself conspicuously when the anthracite is of a type having a relatively low fuel ratio of from about 4 to 10. No technique has yet been established for the production of reduced pellets from the mixture of anthracite having such a low fuel ratio as shown in Table 1.
A main object of the present invention is to provide a method for the manufacture of reduced pellets by use of anthracite having a low fuel ratio without entailing heavy consumption of power such as in the operation of said reduction and without involving any accelerated wear of the equipment and machinery used for the production.
Another object of the present invention is to provide a method for the manufacture of reduced pellets by use of anthracite having a low fuel ratio, said reduced pellets being such that the formed pellets will not disintegrate while they are being roasted for the purpose of reduction or the formed pellets enjoy high strength.